1. Field of the Invention
The present invention relates to passive ring resonators utilizing counter-rotational beams of electromagnetic energy to sense the rate of rotation of such resonators through the shift in resonant frequency induced by the rotation.
2. Description of the Prior Art
Ring resonators of the prior art normally include an active lasing medium disposed therewithin to direct light waves emitted in the active medium in opposite directions around a closed loop, normally planar path. The counter-rotating light beams form an effective standing wave pattern and oscillate at the same frequency when their respective path lenghts are equal and at different frequencies when the path lengths are unequal because of rotation of the path loop about an axis generally perpendicular to the propagation plane of the light waves. Through the detection of this frequency difference, the active ring laser of the prior art is enabled to sense the rate of rotation of its path loop and has thus found wide acceptance in navigation and guidance applications. The prior art ring laser rate of turn device is particularly well suited to modern navigation and guidance systems employing high speed data processing because it can be turned on very rapidly and does provide virtually instantaneous response to input rotation rate changes. Typical practical forms of such ring laser devices are disclosed in the following U.S. Pat. Nos. assigned to the Sperry Corporation.
C. C. Wang--3,382,758 for a "Ring Laser Having Frequency Offsetting Means Inside Optical Path", issued May 14, 1968, PA1 W. M. Macek--3,382,759 for a "Ring Laser Biased by Zeeman Frequency Offset Effect for Sensing Slow Rotations", issued May 14, 1968, PA1 W. M. Macek--3,382,760 for a "Coherent Light Frequency Difference Sensor", issued May 14, 1968, PA1 W. M. Macek--3,508,831 for a "Ring Laser Having Minimized Frequency Locking Characteristic", issued Apr. 28, 1970, PA1 W. M. Macek--3,486,130 for a "Ring Laser Having a Quarter Wave Plate for Rotating the Plane of Polarization of Light Which is Reflected Back into the Ring From the Combining Optics", issued Dec. 23, 1969, and PA1 W. M. Macek--3,480,878 for a "Ring Laser With Means for Reducing Coupling to Backscattered Waves", issued Nov. 25, 1969.
Certain of the foregoing patents, among others, testify to the fact that such active ring rate of turn laser devices, i.e., those having an internal gain mechanism within the ring, demonstrate undesired mode locking phenomena proximate the zero rate of rotation situation. In this condition, the measured output frequency undesirably remains constant over a small range of rates of rotation including and on each side of zero rate of turn. Accordingly, measures must be taken to overcome this effect, such as by the use of mirrors in the laser ring resonator designed to minimize back scatter and by the use of rate biasing techniques which induce a significant difference in frequency between the counter-rotating beams, preventing mode locking at the zero rate of turn condition.
Passive ring rate of turn measurement devices, i.e., those containing no active gain element within the measuring ring or loop, have been recently demonstrated, as reported by S. Ezekiel and S. R. Balsamo, "Passive Ring Resonator Gyroscope", Applied Physics Letters, Vol. 30, page 478 (1977). Related material is presented by the S. Ezekiel U.S. Pat. No. 4,135,822, issued Jan. 23, 1979 for a "Laser Gyroscope". Since the passive ring device does not include a gain medium within its passive ring, some of the problems associated with the presence of such a gain medium are easily avoided. However, prior art passive ring resonator devices have used detection schemes wherein each of the counter-rotating beams is measured and, by means of one or more closed loop feedback schemes, the frequency characterizing at least one of the counter-rotating light beams is adjusted to a predetermined value. There is no attempt to generate a useful rotation rate output signal until this adjustment is accomplished by the feed back apparatus. While fast acting components for such feedback servo schemes are available, the noise introduced in practice by the use of such components increases with increasing speed of response, introducing a signifcant source of error into the rate of turn system. Accordingly, prior art passive resonator rate of turn devices do not match the capabilities of high speed data processor systems normally employed in precision navigation. Furthermore, the need to adjust the frequency of at least one of the light beams in the ring requires interaction with frequency adjusting elements which may in themselves drift due to environmental conditions, injecting further errors into the output rate of turn signal.